System for multi-standard RFID tags

ABSTRACT

A reader for a radio frequency identification system capable of simultaneously reading tags operating a multiple frequencies. The reader includes a radio frequency module for each operating frequency of the tags. The radio frequency modules are coupled to a bus which is connected to an interrogator control module. Each of the radio frequency modules receives the return signal from the tags at the associated operating frequency and converts the return signal into a pulse sequence. The interrogator control module locks to and decodes the pulse sequence according to the protocol associated with the tag type.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a CON of U.S. patent application Ser. No.09/477,478, filed on Jan. 06, 2000, which is U.S. Pat. No. 6,617,962.

FIELD OF THE INVENTION

The present invention relates generally to radio frequencyidentification systems, and more particularly, to a reader for a radiofrequency identification system that can operate with different tags atdifferent frequencies using different protocols.

BACKGROUND OF THE INVENTION

In general an RFID tag system allows for objects to be labeled with tagssuch that when the tag is passed through the electromagnetic field of areader/interrogator the object can be identified by reading the tag thatis attached to the object. In use, RFID tags are attached in a widevariety of methods including being bolted to the item or simply glued tothe inside of existing packaging or labeling. They can be encoded with auser-defined data at time of use, or pre-coded at time of tagmanufacture numbering system or even a combination of both.

Radio frequency identification systems provide a number of advantagesover paper and ink labels, such as bar code systems in that: a muchgreater degree of automation is permitted; clear line of sight is notrequired, tags can be obscured by dirt, paper, even other objects orpackaging; reading distances can be greater; tags can be hidden eitherto protect the tag from damage in use or for security reasons; and inthe case of read/write tags incremental information can be stored on thetags such as PO#, expiry date, destination, confirmation of an appliedprocess, etc.

Those are just some of the advantages of RFID tags. The tag may be asingle integrated circuit chip bonded to a flat, printed antenna, orcould be a complex circuit including battery and sensors fortemperature, position, orientation or any other required feature.

Specifically there are a great deal of different tag types that can becharacterized as having one or more, but not limited to the followingproperties: passive, having no battery and therefore receiving all ofits power required for operation from an electromagnetic fieldtransmitted by the reader/interrogator or active using a self containedbattery on the tag; collision arbitration, meaning that more than onetag can be read in the field of a single reader/interrogator at one timeor non collision, meaning that only one tag can be in the field of thereader/interrogator at a time in order to insure a good read; multiplefrequency where the data from the tag is carried on a differentfrequency from the data to the tag or single frequency where the carrierin both directions is the same; full duplex, where the tag istransmitting data back to the reader/interrogator while thereader/interrogator's transmitter is active or half duplex where the tagwaits for the reader/interrogator's transmitter to go inactive beforereplying; solicited, where the tag must be commanded by thereader/interrogator before it transmits the data back, or unsolicited,where the tag transmits back as soon as it is powered up; activetransmitter, where the tag has its own oscillator and transmitter orback-scatter, where the tag modulates the field set up by thereader/interrogator's transmitter; read only tag, which can be equatedto an electronic barcode or read/write tag, which allows for theequivalent of a scratch pad on the tag. In either case tags can havedifferent sizes of data that is transferred, different sizes ofwrite-able memory, different accessing schemes to the data and differentmethods of writing; carrier frequency, is a function of the application,the physics of the objects being tagged, the range required and theradio frequency spectrum regulations of the country in which it isoperating; data rate, is a function of the carrier frequency, theapplication needs and the radio frequency spectrum regulations of thecountry in which it is operating; data encoding methods can varysignificantly however some form which encodes the data with the clock,such as Manchester encoding is generally used; packet protocol for datatransmission from and to the tag has to be defined in terms of headers,addressing, data field types and sizes, commands, functions,handshaking, etc. etc.; error correction or detection codes, can be usedby the tags to improve reliability of the tag data transfer, generally aCRC error detection only scheme is used, however the particular CRC codemust be specified; additional signaling devices such as beepers or LEDscan be added to the tag to alert and direct the operator to a particulartagged object in the field; additional sensors, such as, for exampletemperature, can be added to the tag to record extreme conditions thatthe tagged object has been passing through.

As can be seen from the list above, there is an extremely wide varietyof tag types that may be used or required by an application making itvery hard to have one reader/interrogator handle all tag types.Typically there would have to be a specific reader/interrogator matchedto the specific properties of each type of tag being used in theapplication.

For example, a typical low cost passive tag system with unsolicited tagresponse, would be implemented as follows; the reader/interrogator wouldfirst activate the tag by generating an electromagnetic field of a givenfrequency. Such an electromagnetic field can be generated, for example,by applying an alternating electrical current at a given frequency to acoil for low frequency near field systems commonly called inductivelycoupled systems or to an RF antenna for far field higher frequencysystems.

The tag includes an antenna, which could be a dipole for far fieldsystems or a coil for inductive systems tuned to the frequency of theinterrogator's generated electromagnetic field. The electrical currentthus generated in the tag's antenna is used to power the tag. Data isgenerally sent to the tag by modulating this interrogator generatedelectromagnetic field which is commonly called the exciter orilluminating field. The tag can send data back to the interrogatoreither by transmitting with its own transmitter with a separatefrequency and antenna from the illuminating field or by modulating theilluminating field by changing the loading of the tag's antenna in whatis commonly called a back scatter system. In any case, either the newelectromagnetic field from the tag or the disturbances in theinterrogator's illuminating field caused by the tag's back scattersystem is detected by the interrogator. The data from the tag is thusdecoded, thereby enabling the tag and the item to which the tag isattached to be identified. In some cases written to, as in the case ofread/write tags by modulating the interrogator's generatedelectromagnetic field. Typical information that might be stored on thetags would be: PO#; expiry date; destination; confirmation of an appliedprocess, etc.

The advantages and disadvantages of using different properties for thetag depend so heavily on the type of application that at this pointthere is no clear winner type of tag that will totally dominate thefield. For example, in some cases range is an advantage, in other casesrange is a disadvantage. Objects with high moisture or water content arenot suitable for tagging with high frequency tags. Applicationsrequiring high data rates or many tags in the field at any one time arenot suitable to low frequency tags. Cost of the tag in relationship tothe object being tagged and or the re-usability of the tag is a veryimportant constraint in selecting tag properties.

As can be seen even from the few examples shown above, any applicationwill be a compromise of tag properties in order to meet theapplication's need. In order to maximize the performance and meet thecost goals, the type of tag must be selected to match the application.Even if a single carrier frequency can be selected for an applicationdifferences in the other properties of the tag could still necessitatedifferent reader/interrogators for the different tag types. Given thatthis is the case and that any large application may have differentperformance goals and therefore tag types, it is extremely advantageousto have a reader/interrogator that is flexible and can read many tagtypes simultaneously. This might even be mandatory in applications wherethere are different reader/interrogator types operating at the samecarrier frequency and thus interfering with each other. Such a universalreader/interrogator would also solve the other great hurdle inimplementing RFID tag systems, and that is the fear of obsolescence andnot being able to read the next type of tag that may be required in theapplication.

In some situations, it is possible for an end user of the radiofrequency identification system to include multiple readers, so thatdifferent tags using different protocols can be read. However, this isinefficient and expensive, as multiple readers would not be required ifa single common standard for tags were used. Furthermore, multiplereaders are likely to interfere with each other, especially if theyoperate at common radio frequencies.

Prior art readers for radio frequency identification systems have beendevised to address some of the above-mentioned problems. For example,International patent application No. PCT/US98/10136, filed by AVIDIdentification Systems, Inc., on 14 May 1998, and entitled READER FORRFID SYSTEM discloses a reader for reading tags of different protocolsin a radio frequency identification system. According to this system,the identification signal from the tag is sensed by the inductive coilof the reader as described above in that the voltage across the coil ismodulated in accordance with the code sequence programmed into the tag.The signal received by the coil is sent to a central processing unit forprocessing and decoding, where the signal is first analyzed by measuringthe pulse width of the signal. The central processing unit then selectsa tag protocol that is most likely to be the correct protocol based onthe pulse width that has been measured.

The AVID radio frequency identification system may suffer from a numberof shortcomings. For example, while the radio frequency identificationsystem provides for reading of tags using different protocols in thesame frequency range, it does not permit tags operating at differentfrequencies to be read by the same reader as the inductive coil of thereader is not operable for all electromagnetic frequencies. The AVIDsystem is essentially an inductive based arrangement operating at asingle frequency. Furthermore, the AVID system does not accommodate allof the tag properties and characteristics described above. Because theAVID system measures a single pulse width, at worst the system can onlyinfer data rate from the pulse width and at best the system can onlyselect from a very small group of tag types where the tag type wouldonly be suitable if it has a distinguishing header pulse width. Ingeneral, the AVID system is not suitable for multiple carrierfrequencies.

In view of the foregoing, there still remains a reader for a radiofrequency identification system that may be used with tags operating atdifferent frequencies with different protocols.

SUMMARY OF THE INVENTION

The present invention provides a reader/interrogator for a radiofrequency identification system which is suitable for use with tagsoperating at different frequencies and also with different tag operatingproperties such as data protocol, encoding, data rates, andfunctionality as introduced above.

The reader/interrogator system according to the invention divides theproblem of multiple tag types into two classes. The first class ischaracterized by carrier frequency and the second class is characterizedby the tag operating parameters. The first class may be broadly brokendown into four principal frequency bands that are in common use today.Each of these bands, on its own, requires its own antenna configuration,transmitter and receiver appropriate to the frequency of operation. Thisfrequency dependent component is referred to as an RFM or radiofrequency module.

The second class is defined as the remaining tag operating parameters,sometimes grouped together and referred to as protocol, and areconsidered as computational problems. This is handled by anothercomponent of the invention referred to as the ICM or interrogatorcontrol module. This module either directly calculates the parametersfrom the incoming tag signal, such as data rate, message length andencoding scheme or exhaustively tries either in parallel or serial thepossible remaining parameters, such as type of CRC used. The results ofthe parameter determinations are verified against a list of acceptabletag parameter combinations before passing on the decoded data as a validmessage.

The reader/interrogator according to the invention simultaneouslyhandles tags operating at different carrier frequencies by utilizing aseparate RFM for each required carrier frequency connected to an ICM.The data being passed between the RFM and ICM is stripped of any carrierfrequencies and is processed by the ICM in a like manner regardless ofwhich frequency band the tag is operating in. The carrier frequency orRFM from which the tag data is received is only used as one of manyparameters to specify a tag type from the list of valid tag typeparameter combinations.

In addition, multiple RFMs operating at the same carrier frequency maybe used with a single ICM where the application requires a specialshaping of the field or multiple antenna orientations or polarizationsin order to read all the tag configurations. In this case the single ICMremoves any problems of interference that would arise from having twoseparate reader/interrogators trying to handle the collision arbitrationand commands to a tag that might be picked up by both unitssimultaneously. It also prevents having the strong signal from onereader/interrogator totally wiping out any low level return signal froma tag which would otherwise only be visible to anotherreader/interrogator.

In accordance with one aspect of the present invention, there isprovided an interrogator for a radio identification system having anumber of tags, with selected tags operating at a first frequency, andother tags operating at another frequency, the interrogator comprises:(a) a first radio frequency module having a transmitter for transmittingan output signal at the first frequency to the tags, and including areceiver for receiving return signals transmitted by the tags operatingat the first frequency; (b) a second radio frequency module having atransmitter for transmitting an output signal at the second frequency tothe tags, and including a receiver for receiving return signalstransmitted by the tags operating at the second frequency; (c) acontroller module coupled to the first and second radio frequencymodules, the controller module including a controlling for controllingthe transmitters for transmitting the output signals to the tags, andincluding a decoder for decoding the return signals received from thetags.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings, which show a preferred embodiment of the present invention,and in which:

FIG. 1 is a block diagram showing a reader according to the presentinvention for a radio frequency identification system;

FIG. 2 is a block diagram showing a conventional tag suitable for usewith the reader according to the present invention;

FIG. 3( a) is a block diagram showing a reader frequency module for thereader according to the present invention;

FIG. 3( b) is a block diagram showing an interrogator control module forthe RFID reader according to the present invention;

FIG. 4( a) is a schematic diagram showing in more detail the front-endof the reader frequency module of FIG. 3( a); and

FIG. 4( b) is a schematic diagram showing in more detail the rear-end ofthe reader frequency module of FIG. 3( a).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is first made to FIG. 1 which shows amultiple-frequency/protocol RFID tag reader according to the presentinvention and indicated generally by reference 10. Themultiple-frequency/protocol RFID tag reader 10 provides the interrogatorin a radio frequency identification (“ID”) system 1. As shown in FIG. 1,the radio frequency identification system or RFID 1 comprises aplurality of tags. In conventional RFID systems the tags in the fieldfor a reader all operate at the same frequency with the same tagparameters. As will be described, the reader 1 according to the presentinvention is suitable for interrogating tags operating at differentfrequencies in the radio frequency field.

The reader or interrogator 10 as shown in FIG. 1 is operable with fourdifferent frequency types of tags 2, 4, 6 and 8. The first type of tags2, shown individually as 2 a, 2 b and 2 c, operate at a first frequency,for example, 125 KHz. The second type of tags 4, shown individually as 4a, 4 b, and 4 c, operate at a second frequency, for example, 13.56 MHz.The third type of tags 6, shown individually as 6 a, 6 b, 6 c, 6 d and 6e, operate at a third frequency, for example, 869 MHz. The fourth typeof tags 8, shown individually as 8 a, 8 b and 8 c, operate at a fourthfrequency, for example, 2.45 GHz. It will be appreciated that while thereader 10 according to the present invention is described in the contextof four types of tags, the reader 10 is suitable for operation with tagsoperating at other frequencies and with differing operating parameterswhether at the same or different frequency.

In the industry, radio frequency identification tags generally come infour different frequency bands, 100–200 KHz., 13.56 MHz., 450-869-917MHz. and 2.45 GHz. As will be understood, all four bands have differentphysical properties which make the tags suitable for specificapplications and environments.

The first frequency band, i.e. 100–200 KHz., is suitable for taggingcontainers holding liquids and also for tagging the human body. Theseradio frequency fields can be well defined and well contained. The firstfrequency band, however, is suitable only for short ranges, typicallyless than one meter. Moreover, the first frequency band is only capableof very low data rates and therefore provides poor performance inapplications requiring multiple tags to be read in the radio frequencyfield at the same time.

The second frequency band, i.e. 13.56 MHz., is commonly used for shortrange passive tags. It is generally inductively coupled in the tag sincethe wavelength is too long for a practical far field antenna. Like thefirst band, the range is relatively short, approximately one meter. Thisfrequency band is also sensitive to the presence of water and de-tunedby the human body.

The third band, i.e. 458-869-917 MHz., is commonly used for long passivetags (e.g. half-duplex tags). The wavelength in this band is shortenough to use dipole antennas and far field effects. This band issuitable for long range tag applications, e.g. one-half watt of powerprovides an approximate range of 10 to 15 feet. This band also supportshigh data rates and with anti-collision algorithms in the reader,numerous tags can be supported at the same time. However, spacers orspecial antennas must be utilized to tag metal objects, and these tagsare not suitable for tagging people or containers of liquid.

The fourth band, i.e. 2.45 GHZ., can support very high data rates and istherefore suitable for multiple tags operating in the radio frequencyfield. Also with the high frequency, only a very small antenna geometryis needed which results in a small footprint for the tag. When comparedto the other frequency bands, tags for this band are the most sensitiveto water and people. Another disadvantage is that these type of tagstend to utilize expensive components in order to provide efficient fieldoperation.

Reference is made back to FIG. 1. To provide the capability to operatewith different types of tags 2, 4, 6 or 8, the reader 10 according tothe present invention comprises an interrogator control module 11, and aradio frequency module for each different frequency of tag. Differingtypes of tags, which have the same carrier frequency, may use the sameradio frequency module. As shown in FIG. 1, the reader 10 includes aradio frequency module 12 for reading the tags 2 operating at the firstfrequency (e.g. 125 KHz.), a radio frequency module 14 for reading thetags 4 operating at the second frequency, a radio frequency module 16for reading the tags 6 operating at the second frequency (e.g. 13.56MHz.), a radio frequency module 16 for reading tags 6 operating at thethird frequency (e.g. 869 MHz.), and a radio frequency module 18 forreading tags 8 operating at the fourth frequency (e.g. 2.45 GHZ.). Theradio frequency modules 12, 14, 16, 18 provide the radio interfacesbetween the respective tag types and the interrogator control module 11.

As shown in FIG. 1, the radio frequency modules 12, 14, 16, 18 arecoupled to the interrogator control module 11 through a bus 19. The bus19 is implemented as a low speed bus and provides control signals to theradio frequency modules 12 to 18 for interrogating the respective typesof tags 2 to 8 and data signal for receiving information transmitted bythe tags to the respective radio frequency modules. By utilizing such anarrangement, the radio frequency modules are arranged in parallel toprovide a multi-frequency capability for the reader 10, which is furtheradaptable by adding additional radio frequency modules or replacing oneor more of the existing radio frequency modules 12, 14, 16 or 18 withradio frequency modules configured for other frequency bands. Inaddition, a plurality of radio frequency modules may be used at the samefrequency band where that might be required for shaping of the field orfor handling different tag orientations being presented in the field. Itwill be appreciated that if multiple radio frequency modules are beingused in the same frequency band, they should differ in the centerfrequency sufficient to meet regulations and such that the beatfrequency between the two units is higher than the maximum data rate.

Reference is next made to FIG. 2, which shows in diagrammatic form theorganization of a typical tag 20 according to the art. The tag 20comprises a series of modules including an air interface 21, logic 22, apower supply 24. If the tag 20 is a read/write tag, there is memorymodule 26. The air interface 21 provides a radio frequency communicationinterface to the reader 10. The logic 22 comprises conventional logic(i.e. digital circuitry) that controls the other modules in the tag 20.The power supply 24 provides local power to run the tag 20. In themajority of tags, i.e. passive tags, the power supply 24 is energized bythe RF signal received from the reader 10. In active tags the powersupply circuit comprises a battery and an activation circuit. If the tag20 is read/writeable, then user defined data may be stored in thismemory and read back by the reader. Depending on the tag properties, aparticular tag might be a write once device or it might be erasable andrewritten many times (typically 10,000). Some tags may only bewrite-able via direct contact and not through the RF interface, however,the reader provides the capability to write the tags via the RFinterface.

Referring next to FIGS. 3( a) and 3(b), the radio frequency module 12and the interrogator control module 11, respectively, are shown in moredetail. Fig. 3( a) also includes tag(s) 2, 4, 6 or 8. According to thisaspect of the present invention, the radio frequency module 12 providesthe radio interface to the associated types of tag (s) 2, 4, 6 or 8. Theradio frequency module 12 is a frequency dependent device, e.g. 100–200KHz., 13.56 MHz, 458-869-917 MHz. or 2.45 GHz. The radio frequencymodule 12 and the tag(s) go together as one type of unit (indicated as13 in FIG. 3 (a), i.e. any given tag frequency will have a dedicatedradio frequency module 12 in the reader 10. As shown in FIG. 3( a), theradio frequency module 12 comprises an air interface stage 31 and a datainterface stage 32. Both the air interface stage 31 and the datainterface stage 32 comprise analogue circuitry as will be described inmore detail below with reference to FIG. 4( a) and 4(b). The datainterface stage 32 provides a data shaping function.

The interrogator control module 11 as described above connects to andcontrols several types of radio frequency modules 12, 14, 16 and 18 andtag types through the bus 19 (FIG. 1). This arrangement according to thepresent invention allows the reader 10 to read tags in the same fieldwhich operate at different frequencies and/or different operatingparameters. As shown in FIG. 3( b), the interrogator control module 11comprises a data interface and protocols stage 34, and an applicationinterface stage 36.

The application interface stage 36 comprises a programmed microprocessorwhich interfaces to the data interface and protocols stage 34 andcontrols the operation of the interrogator control module 11 and theindividual radio frequency modules 12, 14, 16 and 18 through the bus 19.As shown in FIG. 3( b), the interrogator control module 11 also includesa LCD touch panel 38 for accepting user commands and displayinginformation concerning the operation of the radio frequencyidentification system 1, the radio frequency modules 12, 14, 16 and 18,and the tag types. Preferably, the program memory for the microprocessorin the application interface 36 is implemented using flash memorythereby allowing programs to be downloaded from a PC (not shown) via aconventional network connection.

The data interface and protocols stage 34 includes circuitry forprocessing the receive signal output from the data interface stage 32(FIG. 3( a)) in the radio frequency modules 12 (14, 16 and 18). Thisprocessing includes performing clock separation, recovering data fromthe receive signal output, and the handling of data protocols based thetag types being controlled. The data interface and protocols stage 34 ispreferably implemented as a field programmable gate array or FPGA.Advantageously, an implementation utilizing a field programmable logicdevice allows the reloading of different protocols under the control ofthe microprocessor in the application interface 36. The FPLD isprogrammed to accept the data rates and protocols available on thevarious types of tags 2, 4, 6 or 8. In operation, the microprocessor inthe interrogator control module 11 loads the FPGA with the appropriateconfiguration data to handle data decoding and protocol conversion forthe tags which are to be interrogated in the field. At this point thedata is totally stripped of its carrier frequency component and otherthan being used as an index into a list of acceptable tag types withtheir possible operating parameters, the carrier frequency is no longerused in the decoding. There are tag families which use the same logiccircuit and therefore operating parameters regardless of the carrierfrequency. In this case the data interface and protocol stage use thesame procedure for decoding regardless of which RFM the signal came inon. In some cases there will be different tag types operating on thesame carrier frequency and the interface and protocol stage will usedifferent procedures even for signals coming in on the same RFM.

The FPGA directly controls the transmitter for data going back to thetag or for collision arbitration signals going to the tags, since theFPGA has derived the clock rate and timing required for the particulartag type. The RFM and the microprocessor may also gate these signals tohave general control of the RFM's transmitter. The ICM turns on thetransmitters according to regulatory and application requirements topower the passive tags and/or to wake up the active tags and any pollingsequence that may be required for the tags types in use is transmitted.The ICM then waits for the response signals from the tags and determinesthe type of tag that is in the field. The FPGA directly calculates aselected parameter from the incoming tag signal, such as data rate,message length and encoding scheme or exhaustively tries either inparallel or serial the possible remaining parameters, such as type ofCRC used. The results of the parameter determinations are verifiedagainst a list of acceptable tag parameter combinations before passingon the decoded data as a valid message.

The FPGA is configured to handle all low level communications to andfrom the tag via the air interface stage 31 and the data interface stage32 in the radio frequency module 14. While the FPGA is programmed tohandle the low level communication, the microprocessor is programmed toperform all higher level data protocol conversions and the forwarding ofprocessed data to the user (i.e. via a LCD touch panel) or to anetworked PC using a standard communication protocol such as TCP/IP.

Preferably, the handling of data rate and data encoding for the tags inthe reader 10 is implemented as a clock and data separation schemeutilizing a phase locked loop on the incoming signal. Thisimplementation is advantageous since the tag rate can and will varyduring transmission and therefore measuring a single pulse is generallynot sufficient to yield an accurate bit rate for the tag data. Thisyields a far better result than just measuring the width of the leadingpulse in the message.

It will be understood that while accurately determining the data rate ofthe tag message may be sufficient to distinguish between the tag typeson the basis of different data rates, in general, this is not sufficientto determine other operating parameters of the tag.

The data and clock separation function in the FPGA presents the data tothe protocol and error checking function of the FPGA. Preferably theFPGA is implemented to provide several protocols and CRC checks inparallel. The path leading to a full check, or zero errors in decodingis assumed to be the correct operating parameters for that tag. The tagmessage along with the assumed tag type is presented to themicroprocessor which then determines if the type is in the list ofacceptable tag types. If so the tag data is passed on to theapplication.

Reference is next made to FIGS. 4( a) and 4(b) which show in more detailthe air interface stage 31 and the data interface 32, respectively, fora radio frequency module 12, 14, 16 and 18. In particular the figuresdepict a far field effect type of RFM which uses an RF antenna asopposed to a coil with inductive coupling. This type of RFM is suitablefor the high frequencies such as UHF or higher. As shown in FIG. 4( a),the air interface stage 31 comprises an antenna 101, a circulator 102, atransmitter 104, a mixer stage 106, and an amplifier stage 108.Preferably, the mixer stage 106 and amplifier stage 108 comprise aminimum of two channels with a delay between them to allow forquadrature decoding of the signal. Additional channels and delays couldbe added to allow the same RFM to be used at different frequencies. FIG.4( a) shows three channels, which could allow for quadrature decoding ofup to three separate carrier frequencies. Each channel having acorresponding mixer 107, shown individually as 107 a, 107 b and 107 c,and a corresponding amplifier 109, shown individually as 109 a, 109 b,and 109 c. As will be understood by those skilled in the art, the threechannel configuration allows quadrature information to be extracted foreach tag. The transmitter 104 is coupled to the antenna 101 through thecirculator 102. In known manner, the circulator 102 allows the antenna101 to be used for both transmitting signals to the tags 2 (4, 6, and 8)and receiving signals from the tags 2 (4, 6, and 8). The transmitter 104generates a constant field (i.e. the illumination or power signal) whichprovides power to each of the tags 2 associated with the radio frequencymodule 12. The transmitter 104 also generates a reference output signalthat is fed to the mixer for each of the channels as shown in FIG. 4(a). The mixer 107 b for the second channel includes a delay element 105a to delay the feed of the reference output signal. Similarly, the mixer107 c for the third channel includes a delay element 105 b to furtherdelay the feed of the reference output signal (received from the firstdelay element 105 a).

The mixer subtracts the carrier to produce a Non Return to Zero image ofthe data that was modulated onto the carrier by the tag. The signal fromthe mixer is AC coupled to the amplifier stage to remove any DCcomponent that might be contained in the signal. This allows for highergains on the amplifiers. The signal is then further differentiated toprovide sharp pulses on the leading and trailing edges of the data bits.Advantageously, this allows for even higher amplification stages andeliminates the need for filtering between stages according to the datarate, thus making the channel suitable for a wide range of data rates.The data interface stage 32 receives the output from the amplifier stage108 (i.e. the amplifier 109 for each of the three channels) in the airinterface stage 31. The data interface stage 32 provides a pulse shapingoperation and comprises a pulse shaping circuit 110 as shown in FIG. 4(b) for each of the three channels in the air interface stage 31 (FIG. 4(a)). The pulse shaping circuit 110 comprises a discriminator 111, arectifier circuit 112, a summing amplifier 114, a logic level convertor116 and a output port 118. The discriminator 111 comprises a capacitorwhich couples the output from the amplifier 109 a to the rectifiercircuit 112. The discriminator 111 converts the output signal intopulses with defined edges. The rectifier circuit 112 comprises a pair ofdiodes which separate the pulses into positive and negative edges. Thepositive and negative edges are then summed together by the operationalamplifier 114 resulting in a pulse for each edge. The logic levelconvertor converts the level of the pulses for output. The output port118 to the bus 19 (FIG. 1) is implemented using an opto-coupler devicewhich advantageously provides isolation and level conversion between thecircuitry in the radio frequency module 12 (14, 16 or 18) and theinterrogator control module 11. The pulses are then processed by a dataprotocol decoder in the data interface and protocols stage 34 of theinterrogator control module 11.

The data decoder is implemented in the program (i.e. firmware) executedby the microprocessor in the data interface and protocols stage 34. Thedata decoder provides the functionality to decode the pulse streamsreceived from the data interface stage 32 in the radio frequency module12 (14, 16 and 18). Utilizing the phase locked loop clock and dataseparation scheme, the pulse stream is decoded according to the protocol(e.g. Manchester encoded)associated with the type of tag 2, 4, 6 or 8.The three output data channels in each radio frequency module 12, 14, 16and 18 provide parallel paths for the decoding the data received fromthe tags. The programmed microprocessor performs code checking and CRCdecoding to select the tag data stream which does not have any codeviolations and a successful CRC result.

In operation, the interrogator control module 11 initiates theinterrogation of the tags through the radio frequency module configuredfor the frequency band of the tags, for example, the second radiofrequency module 14 (FIG. 1) is configured for the 13.56 MHz tags 4. Theinterrogation can be in response to an input from the user entered onthe LCD touch panel 38 or to a command received from a networked PC. Theinterrogator control module 11 sends a command (i.e. control signals)via the bus 19 to the radio frequency module configured for the tagsbeing interrogated, for example, the radio frequency module 14 for 13.56MHz tags 4. In the radio frequency module 14, the transmitter 104 (FIG.4( a)) excites the antenna 101 to transmit an interrogation or powersignal to the tags 4 in the field. At the same time, the transmitter 104also generates a reference output signal for the mixer stage 106 (FIG.4( a)) as described above. The tags 4 tuned to the frequency of theradio frequency module 14 receive and are energized by the interrogationsignal and after a short delay transmit their response signals back tothe radio frequency module 14. The response signals are received by theantenna 101 and reflected back and split into three channels for themixer stage 106 through the circulator 102. In the mixer stage 106, thereceived signal is subtracted from the transmitted signal to produce aphase shift. The output from the mixer stage 106 is passed to theamplifier stage 108. The amplified signal is then shaped by the pulseshaping circuit 110 (as described above with reference to FIG. 4( b)) togenerate a series of pulses in three channels. The three channels ofpulses are transferred over the bus 19 to the data interface andprotocols stage 34 of the interrogator control module 11. In theinterrogator control module 11, the data decoder (data interface andprotocols stage 34) uses a phase locked loop clock and data separationscheme to lock onto the pulse stream, determine the protocol for thetag, and extract the data transmitted by the tag. Code checking andapplication of CRC is also performed to ensure the integrity of the datadecoding.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Certainadaptations and modifications of the invention will be obvious to thoseskilled in the art. Therefore, the presently discussed embodiments areconsidered to be illustrative and not restrictive, the scope of theinvention being indicated by the appended claims rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

1. An interrogator for a radio identification system having a pluralityof tags, the plurality of tags including a first subset of tagsoperating only within a first frequency band, the plurality of tagsincluding a second subset of tags operating only within a secondfrequency band, the first subset of tags excluding each tag from thesecond subset of tags, said interrogator comprising: (a) a first radiofrequency module having a transmitter configured to transmit an outputsignal at a first frequency to the first subset of tags, the first radiofrequency module including a receiver configured to receive returnsignals transmitted by the first subset of tags operating at the firstfrequency, the first frequency being within the first frequency band,the transmitter and the receiver of the first radio frequency modulebeing operable over the first frequency band, the first frequency bandincluding a plurality of frequencies each being outside the secondfrequency band; (b) a second radio frequency module having a transmitterconfigured to transmit an output signal at a second frequency to thesecond subset of tags, the second radio frequency module having areceiver configured to receive return signals transmitted by the secondsubset of tags operating at the second frequency, the transmitter andthe receiver of the second radio frequency module being operable overthe second frequency band, the second frequency band including aplurality of frequencies each being outside the first frequency band;(c) a controller module coupled to said first and second radio frequencymodules, said controller module including a controller configured tocontrol the transmitter associated with the first frequency and thetransmitter associated with the second frequency, the controller moduleincluding a decoder configured to decode return signals received fromsaid tags.
 2. The interrogator of claim 1, wherein the decoder includesa signal divider configured to divide the return signals into multiplechannels and a converter configured to produce pulses based on thereturn signals.
 3. The interrogator of claim 2, wherein the decoderincludes a synchronizer configured to synchronize a frequency of thepulses and an extracter module configured to extract information fromthe pulses according to a protocol associated with the tag transmittingthe return signal.
 4. The interrogator of claim 3, wherein the decoderfurther includes a code checker associated with the pulses and selectorconfigured to select the channel without code violations.
 5. Theinterrogator of claim 2, wherein each transmitter includes an antennaconfigured to transmit its output signal in response to a control signalfrom the controller.
 6. The interrogator of claim 5, wherein the dividerincludes a circulator having an input port coupled to the antenna and anoutput port for each of the channels.
 7. The interrogator of claim 6,wherein the divider includes a mixer and an amplifier for each of thechannels, each of the mixers having an input coupled to the respectiveoutput port of the circulator and an output coupled to the respectiveamplifier for the channel, the output of each amplifier is coupled to aconverter configured to convert the return signals into pulses for thechannel.
 8. The interrogator of claim 7, wherein the converter includesa pulse shaping circuit for each of the channels.
 9. The interrogator ofclaim 8, wherein the pulse shaping circuit includes an isolated outputport coupled to a bus connected to the controller.
 10. The interrogatorof claim 1, wherein the first radio frequency module and the secondradio frequency module are coupled to the controller through a bus. 11.The interrogator of claim 10, wherein the plurality of tags including athird subset of tags operating only within a third frequency band, thethird subset of tags excluding each tag from the first subset of tagsand each tag from the second subset of tags, the third frequency bandhaving a plurality of frequencies each being outside the first frequencyband and the second frequency band, the interrogator further comprising:a third radio frequency module having a transmitter configured totransmit an output signal at the third frequency to the tags, the thirdradio frequency module having a receiver configured to receive returnsignals transmitted by tags operating at the third frequency.
 12. Theinterrogator of claim 11, the plurality of tags including a fourthsubset of tags operating only within a fourth frequency band excludingeach tag from the first subset of tags, each tag from the second subsetof tags and each tag from the third subset of tags, the fourth frequencyband having a plurality of frequencies each being outside the firstfrequency band, the second frequency band and the third frequency band,the interrogator further comprising: a fourth radio frequency modulehaving a transmitter configured to transmit an output signal at thefourth frequency to the tags, and including a receiver configured toreceive return signals transmitted by tags operating at the fourthfrequency.
 13. The interrogator of claim 12, wherein the first frequencyfalls in a range 100 to 200 KHz.
 14. The interrogator of claim 13,wherein the second frequency is substantially 13.56 MHz.
 15. Theinterrogator of claim 14, wherein the third frequency falls in a range458 to 917 MHz.
 16. The interrogator of claim 15, wherein the fourthfrequency is substantially 2.45 GHz.
 17. A radio identificationinterrogator, comprising: a first radio frequency (RF) module beingassociated with its own frequency band and having a transmitter and areceiver, the transmitter of the first RF module being configured tosend an output signal having a carrier frequency within the frequencyband associated with the first RF module, the receiver of the first RFmodule configured to receive a return signal based on the output signalassociated with the transmitter of the first RF module and beingassociated with a tag from a first plurality of tags, the first RFmodule is configured to receive a return signal modulated according to afirst protocol from a plurality of protocols; and a second RF modulebeing associated with its own frequency band different from thefrequency band associated with the first RF module, the second RF modulehaving a transmitter and a receiver, the transmitter of the second RFmodule being configured to send an output signal having a carrierfrequency within the frequency band associated with second RF module,the receiver of the second RF module configured to receive a returnsignal based on the output signal associated with the transmitter of thesecond RF module and being associated with a tag from a second pluralityof tags, the first plurality tags excluding each tag from the secondplurality of tags, the second RF module is configured to receive areturn signal modulated according to a second protocol from a pluralityof protocols.
 18. The radio identification interrogator of claim 17,further comprising: a controller coupled to the transmitter of the firstRF module and the transmitter of the second RF module; and a decodercoupled to the receiver of the first RF module and the receiver of thesecond RF module.
 19. The radio identification interrogator of claim 17,further comprising: a decoder coupled to the receiver of the first RFmodule and the receiver of the second RF module, the decoder having afirst output channel associated with the first protocol and the secondoutput channel associated with the second protocol.
 20. A method,comprising: sending a signal having a carrier frequency within a firstfrequency band; sending a signal having a carrier frequency within asecond frequency band mutually exclusive from the first frequency band;receiving, from a second tag operative only within the second frequencyband, a return signal based on the signal having the carrier frequencywithin the first frequency band, the return signal associated with thefirst tag being within the first frequency band and outside the secondfrequency band; and receiving, from a second tag operative only withinthe second frequency band, a return signal based on the signal havingthe carrier frequency within the second frequency band, the returnsignal associated with the second tag being within the second frequencyband and outside the first frequency band.
 21. The method of claim 20,wherein: the return signal associated with the first frequency band ismodulated according to a first protocol from a plurality of protocols;and the return signal associated with the second frequency band ismodulated according to a second protocol from a plurality of protocols.22. The method of claim 20, wherein the carrier frequency within thefirst frequency band is between 100 to 200 KHz.
 23. The method of claim20, wherein the carrier frequency within the second frequency issubstantially 13.56 MHz.
 24. The method of claim 20, further comprising:sending a signal having a carrier frequency within a third frequencyband, the third frequency band being mutually exclusive from the firstfrequency band and the second frequency band; and receiving, from athird tag operative only within a third frequency band, a return signalbased on the signal having the carrier frequency within the thirdfrequency band, the return signal associated with the third frequencyband being associated with the third tag, not the first tag and not thesecond tag, the carrier frequency within the third frequency band beingbetween 458 to 917 MHz.
 25. The method of claim 24, further comprising:sending a signal having a carrier frequency within a fourth frequencyband, the fourth frequency band being mutually exclusive from the firstfrequency band, the second frequency band and the third frequency band;and receiving, from a fourth tag operative only within a fourthfrequency band, a return signal based on the signal having the carrierfrequency within the fourth frequency band, the return signal associatedwith the fourth frequency band being associated with the forth tag, notthe third tag, not the second tag and not the first tag, the carrierfrequency within the fourth frequency band being substantially 2.45 GHz.26. A radio identification interrogator, comprising: a first radiofrequency (RF) module being associated with its own frequency band andhaving a transmitter and a receiver, the transmitter of the first RFmodule being configured to send an output signal having a carrierfrequency within the frequency band associated with the first RF module,the receiver of the first RF module configured to receive a returnsignal within the frequency band associated with the first RF module andbased on the output signal associated with the transmitter of the firstRF module, the return signal associated with the first RF module beingassociated with a tag from a first plurality of tags; and a second RFmodule being associated with its own frequency band mutually exclusivefrom the frequency band associated with the first RF module, the secondRF module having a transmitter and a receiver, the transmitter of thesecond RF module being configured to send an output signal having acarrier frequency within the frequency band associated with second RFmodule, the receiver of the second RF module configured to receive areturn signal within the frequency band associated with second RF modulebased on the output signal associated with the transmitter of the secondRF module, the return signal associated with the second RF module beingassociated with a tag from a second plurality of tags, the firstplurality tags excluding each tag from the second plurality of tags, thefirst frequency band including a plurality of frequencies each beingoutside the second frequency band, the second frequency band including aplurality of frequencies each being outside the first frequency band.